Planar slot array antennas are well known in the art and generally include radiating slot apertures cut in the broad walls of several contiguous waveguides or waveguide cavities that form the aperture, and some means of distributing a radio frequency signal in such a way as to excite each radiating slot with the proper electric field to produce the desired farfield radiation pattern. Planar slot arrays have found wide usage in airborne radar and missile guidance system applications because of their compact size, high efficiency, and economical construction.
When an array is divided functionally into four quadrants, the four signals received can be processed by a passive microwave device known as a "comparator" to provide three new signals, referred to as sum, azimuth difference, and elevation difference. The three signals are generated simultaneously and can act upon a single received radar pulse to provide tracking information--hence the term "monopulse" tracking system. Such tracking systems are widely preferred over antecedent lobing techniques because of their relative immunity to jamming (electronic countermeasure techniques used by an adversary to disrupt the radar system's tracking capability.)
Certain properties of the monopulse antenna are crucial to the operation of the radar system. The detection range of a radar system is proportional to the square root of the antenna gain. In contrast, it is only proportional to the fourth root of the transmitted power or receiver noise figure. Antenna gain can be defined as the antenna efficiency times the maximum theoretical gain that can be achieved for a given sized aperture. The antenna efficiency is a number less than one. Sidelobe level is a measure of the field strength in a given direction relative to that at the peak of the main beam. Low sidelobes reduce unwanted ground clutter or jamming signals entering the antenna from directions other than the main beam.
Properties of the difference pattern are important to the tracking operation. The difference pattern peak level should not be too far below that of the sum peak or the tracking range will be degraded. The null of the difference pattern should be deep and pointed in a direction parallel to the antenna boresight to prevent false error information. As with the sum beam, sidelobe levels must be low. Sensitivity is defined as ratio of difference pattern level to sum pattern level at a specified angle from boresight. Tracking loop gain is adjusted on the basis of this parameter, therefore it is important that the sensitivity be constant over the frequency band of the antenna if accurate radar tracking or missile guidance is to take place.
One problem that has plagued the users and designers of small monopulse tracking arrays is that sum and difference patterns cannot be optimized simultaneously without resorting to slot excitation or "feeding" techniques that are prohibitively complex. Consequently, difference pattern sidelobes are generally marginal in performance. Even more serious is the drawback that monopulse sensitivity variations over a frequency band are excessive; the cause of this problem has previously been poorly understood. An invention which can help to substantially alleviate these problems will be described.
The best known prior art consists of an array of slots in the conducting surface formed by the broad walls of several contiguous waveguides. Each quarter or quadrant of the antenna is connected to a monopulse comparator to provide sum, azimuth difference, and elevation difference signals.
The monopulse comparator consists of 4 sum and difference circuits, sometimes referred to as "hybrid tees" or "magic tees", connected by transmission lines in such a way as to provide the sum of the four antenna quadrant signals A+B+C+D, the elevation difference signal (A+B)-(C+D) and the azimuth difference signal (A+C)-(B+D).
In order to provide low antenna sidelobes for the sum beam, which is used for searching and tracking functions, the distribution of electric field over the aperture is lowest at the edge and highest towards the middle. The sharp discontinuity produced in the aperture field strength at the center of the aperture is generally known to produce high difference pattern sidelobes. The inventors have also found that this discontinuity produces other anomalous performance degradation including a further deterioration of difference pattern sidelobe levels that is not predicted by existing theory and serious fluctuations of the sensitivity and difference pattern peak levels over the operating frequency range of the antenna. This behavior is largely due to changes in impedance of the individual radiator in the presence of other radiators, i.e., mutual coupling.
To improve difference pattern peak levels and sidelobe levels others have devised complex feed systems that produce independent sum and difference excitations so that ideal sum and difference pattern excitations can be simultaneously achieved. Included in this category are ladder feeds and dual corporate feeds. These feed systems suffer from the drawback that their size, weight, cost and complexity render them unattractive for the majority of applications. For applications with extreme weight and volume restrictions, such as missile seeker radars, antennas with these complex feed systems are totally unacceptable.
Other inventions, such as dipole augmented slot radiating elements (U.S. Pat. No. 3,594,806) produce a modest reduction in mutual coupling, relative to simple slot radiators; however, these augmented slots provide insufficient decoupling of the antenna quadrants and introduce increased costs and mechanical problems for manufacturing.